Activation of Ca2+ release in isolated sarcoplasmic reticulum

Summary The relationship between Ca²⁺ release from sarcoplasmic reticulum, induced by elevated pH, tetraphenylboron (TPB^–) or chemical modification, and the change in the surface charge of the membranes as measured by the fluorescence intensity of anilinonaphthalene sulfonate (ANS) is examined. The stimulated Ca²⁺ release is inhibited by dicyclohexylcarbodiimide and external Ca²⁺. TPB^–, but not tetraphenylarsonium (TPA⁺), causes a decrease in ANS^– fluorescence, with 50% decrease occurring at about 5 m TPB^–. The decrease in ANS^– fluorescence as well as the inhibition of Ca²⁺ accumulation induced by TPB^– are prevented by TPA⁺. A linear relationship between the decrease in membrane surface potential and the extent of the Ca²⁺ released by TPB^– is obtained. Similar levels of ³H]TPB^– bound to sarcoplasmic reticulum membranes were obtained regardless of whether or not the vesicles have taken up Ca²⁺. The inhibition of Ca²⁺ accumulation and the ³H]TPB^– incorporation into the membranes were correlated. Ca²⁺ release from sarcoplasmic reticulum, by pH elevation, chemical modification or by addition of NaSCN (0.2 to 0.5m) or the Ca²⁺ ionophore ionomycin, is also accompanied by a decrease in ANS^– fluorescence intensity. However, chemical modification and elevated pH affects the surface potential much less than SCN^– or TPB^– do. These results suggest that the enhancement of Ca²⁺ release by these treatments is not due to a general effect on the membrane surface potential, but rather through the modification of a specific protein. They also suggest that membrane surface charges might play an important role in the control mechanism of Ca²⁺ release.